1. Field of the Invention
The present invention relates to an optical wavelength filter and an optical demultiplexer for a receiver of a wavelength division multiplexing (WDM) transmission system, and more particularly, to an optical demultiplexer for a low-loss, high-density WDM transmission system.
2. Description of the Related Art
A WDM transmission system multiplexes the wavelength area of an optical fiber into several channels by simultaneously transmitting signals of several wavelength bands, relying on the wavelength characteristics of an optical signal. In the WDM transmission system, an input optical signal, having been multiplexed to have several wavelength components, is demultiplexed at the receiver and recognized in the respective channels.
In earlier WDM transmission systems a 1.times.n coupler and first through nth bandpass filters are connected where n represents the number of channels of a transmitted optical signal.
A coupler is a passive device for branching or coupling optical signals and the output power of each branch optical signal is 1/n that of the input optical signal to the 1.times.n coupler. Hence, the power output of each of the n optical signals from the n bandpass filters is 1/n that of the input optical signal.
In order to compensate for this loss of power, it is necessary to include an optical amplifier for amplifying the optical signal to increase the power by n times before it is input to the 1.times.n coupler.
This causes an inconvenience in that the optical demultiplexer for a receiver must have an optical amplifier in order to make for the power loss of the 1.times.n coupler.